1. Field of the Invention
The present invention relates to a developing device that supplies toner from a toner transport member to the surface of an image carrier member having electrostatic latent image formed thereon so as to develop said image with said toner, and more specifically relates to a monocomponent type developing device using a monocomponent developing material.
2. Description of the Prior Art
Conventionally, various types cf developing devices which supply toner to an electrostatic latent image formed on the surface of an image carrier member have been used in electrophotographic copying machines and the like.
Developing device used in electrophotographic copying machines require different developing characteristics depending on whether the original document image consists of a line image or solid image. That is, when the original document image is a solid image, copy image density must be obtained which corresponds to the density of the original document image, whereas when the original image is a line image, the density of the copy image must be higher than that of the original document image density.
In accordance with the aforesaid requirements, two-component type developing devices which use a two-component developing material comprising a toner and a carrier produce a so-called edge effect in line images due to the action of the carrier contained in the developing material, and the copy image density rises rapidly relative to the original document image density, and produces a high copy image density even when the original document image density is low and, when used for solid images, produces a copy image density which corresponds to the original document image density.
However, problems such as image variability and the like occur when using the aforesaid type of two-component developing device due to carrier deterioration and variance in the toner/carrier mixing ratio.
On the other hand, monocomponent developing devices, which use a toner alone as the developing material in a contact type developing method employing a conductive roller or conductive flexible sleeve, produce excellent .gamma.-characteristics and rapid rise in copy image density relative to the original document image density.
Thus, the aforesaid monocomponent type of developing device is suitable for digital image reproduction used by printers and the like, but has poor reproducibility when multistage density gradation is required for digital color images and tone reproduction in copier images.
The previously described monocomponent developing device has been thoroughly investigated and produces a copy image density that corresponds to the original document image density for solid images, and rapidly increases copy image density when the original document image density is low for line images so as to produce a high density copy image.
Japanese Patent Application No. 57-114163 discloses an electrically insulated conductive substrate together with a mutually insulated microelectrodes that are electrically insulated from said substrate by means of an insulating material on said substrate member used as a toner transporting member in the previously described monocomponent type of developing device.
When the aforesaid type of toner transporting member is used, the microelectrodes perform the same role as the carrier in the previously described two-component developing material through the insulating material on the electrically conductive substrate so as to produce the same developing characteristics as the aforesaid two-component type developing device.
However, in developing devices using the aforesaid toner transporting member, the electric lines of force from the electrostatic latent image toward to the electrically conductive substrate are regulated by the thickness and permittivity of the insulating material provided on the conductive substrate of the toner transporting member so as to thereby determine the edge effect, tone and like image characteristics. Although these properties must be regulated to correspond to solid or line original document images to produce suitable edge effect and tone, it is extremely difficult to regulate the aforesaid image characteristics such as edge effect and tone by changing the characteristics of this type of toner transporting member.
In this type of developing device, a means for suitably altering developing characteristics is disclosed in U.S. patent application No. 4564285, wherein is described a layer of electrically insulating material supporting microelectrodes disposed on an electrically conductive layer, or a dielectric layer provided beneath an electrically conductive layer such that the developing characteristics may be changed by applying a bias voltage to either electrically conductive layer.
However, this type of multilayered toner transporting member is very difficult to manufacture, and problems arise from the use of the method of applying a bias voltage to the conductive layer disposed between the dielectric layer and the insulating layer containing the microelectrodes.
Further, the previously mentioned toner transporting member disclosed in Japanese Patent Application No. 57-114163 provides microelectrodes in an insulating material on an electrically conductive substrate, and because the surface of the insulating material loses suppleness and hardens, sufficient adhesion is not obtained during contact between the insulating surface and the hard image carrier such as a photoconductive drum, and the surface of said image carrier may be damaged by such contact, thereby reducing image quality.
When toner is supplied from the aforesaid toner transporting member to the surface of the image carrier, a charge remains on the microelectrodes requiring the use of a discharging means to eliminate said residual charge, and if said residual charge is not sufficiently discharged it accumulates and reduces image quality.
In recent years, therefore, construction of the insulation material supporting the microelectrodes using photoconductive material which is electrically conductive only when exposed to light has been proposed together with the use of an eraser lamp as a discharging means so as to sufficiently discharge the residual charge from the microelectrodes, as disclosed in Japanese Patent Application No. 58-211172.
In the aforesaid construction, however, construction costs are increased and manufacturing is complicated by the need to use special photoconductive material in the insulating material which contains the microelectrodes.